The present invention generally relates to hydraulic control apparatuses for automatic transmissions that are mounted on vehicles and the like. More particularly, the present invention relates to hydraulic control apparatuses for automatic transmissions, which enable a shift speed to be attained even in case of a failure that causes de-energization.
In general, in multi-stage automatic transmissions that are mounted on vehicles and the like, each shift speed is formed by controlling the rotating state of each rotary element of a speed change gear mechanism according to the engagement state of a plurality of friction engagement elements. The engagement state of the plurality of friction engagement elements is controlled by electrically regulating an engagement pressure using solenoid valves, and thus, supplying the regulated engagement pressure to respective hydraulic servos of the friction engagement elements.
Incidentally, if a failure in which no electricity is supplied to the solenoid valves, which is a so-called all-solenoids-off state, occurs in such automatic transmissions, electric shift control using the solenoid valves cannot be performed. Such an all-solenoids-off state can be considered to be caused by, for example, failures of a control unit (ECU), disconnections or short-circuits of battery wires, or the like.
On the other hand, when normally open type solenoid valves, which output an oil pressure when being de-energized, are used as the solenoid valves for regulating the engagement pressure to be supplied to the hydraulic servos of the friction engagement elements, the oil pressure can be supplied to the hydraulic servos in the all-solenoids-off state. However, the use of such solenoid valves has the following problem. These solenoid valves do not output the engagement pressure during normal traveling, and thus, often consume electric power. That is, reduction in power consumption is prevented due to the failure-safe function, thereby hindering improvement in fuel economy as a vehicle.
In view of this problem, a hydraulic control apparatus has been proposed, in which all the solenoid valves are formed by normally closed type solenoid valves, where an oil pressure is reversely inputted from exhaust ports of specific solenoid valves (see Japanese Patent Application Publication No. JP-A-2007-177932). This hydraulic control apparatus is structured so that, if, for example, all-solenoids-off failure occurs during traveling, a forward range pressure can be reversely inputted to exhaust ports of linear solenoid valves SLC2, SLC3 that are connected to a second clutch C-2 and a third clutch C-3 that form the seventh forward speed. This improves the fuel economy in a normal state, and also attains the failure-safe function by forming the seventh forward speed in case of the all-solenoids-off failure.